The present invention relates generally to imaging in the terahertz (THz) frequency range and, more specifically, to a method for improving the resolution of electro-optic terahertz imaging.
The technique of imaging is generally understood as the measurement and replication of the intensity distribution of an active source emitting an electromagnetic wave or the backscattering profile of a passive object or scene. The functionality of imaging can be greatly extended by incorporating spectroscopy techniques in the imaging system. For example, organic functional groups in an imaged specimen can be identified and imaged by their select patterns of absorption wavelength. Microwave imaging and optical imaging are well-known and have been long used in the art.
Compared with the long history of microwave and optical imaging, however, terahertz (THz) wave imaging based on optoelectronic THz time-domain spectroscopy is in its infancy, having only recently emerged within the last six years. THz radiation occupies a large portion of the electromagnetic spectrum between the infrared (IR) and microwave bands, namely the frequency interval from 0.1 to 10 THz. Professor Zhang, a co-inventor of the present invention, holds a number of patents in this field, including U.S. Pat. Nos. 5,952,818, 6,057,928, and 6,111,416, all of which are incorporated in this application by reference for their basic teachings.
THz time-domain spectroscopy (THz-TDS) is based on electromagnetic transients that are generated and detected opto-electrically by femtosecond laser pulses. These THz transients are typically single-cycle bursts of electromagnetic radiation of less than 1-ps duration. These THz transients have a spectral density that typically spans the range from below 0.1 THz to more than 3 THz, and a brightness that typically exceeds greatly that of conventional thermal sources, due to high spatial coherence.
The temporally gated detection technique allows direct measurement of the THz electric field in the time domain with a time resolution of a fraction of a picosecond (ps). The detection is thus xe2x80x9ccoherent,xe2x80x9d meaning that both the amplitude and the phase of the THz spectrum can be extracted from the Fourier transform of the detected THz time-domain waveform. This characteristic is very useful for applications that require the measurement of the real and imaginary parts of the dielectric function. The sensitivity of the gated detection technique is orders of magnitude higher than traditional incoherent detection. In addition to this benefit, time-gated coherent detection is immune to incoherent far-IR radiation, making it possible to perform spectroscopy of high-temperature materials even in the presence of strong blackbody radiation background.
There are two main mechanisms typically employed for the generation of THz radiation in a typical THz-TDS system: photoconduction and optical rectification. In the first, photoconductors switched by an ultrafast laser pulse function as a radiating antenna. Based on their structure, the antennas can be classified as elementary Hertzian dipole antennas, resonant dipole antennas, tapered antennas, transmission line antennas, or large-aperture antennas. For THz generation via optical rectification, electro-optic crystals are used as the THz source. With the incidence of an ultrafast pulse on the electro-optic crystals, the different frequency components within the bandwidth of the fundamental optical beam form a polarization that oscillates at the beat frequency between these frequency components. This time-varying dielectric polarization produces a transient dipole that radiates broadband electromagnetic waves. In comparison with the THz radiation from photoconductive antennas (PDAs), THz optical rectification radiation has less power, but shorter pulse duration and larger bandwidth. The average power level of THz optical rectification radiation can reach several microwatts, depending on the pump power of the ultrafast laser sources.
Free-space electro-optic sampling (FS-EOS) is a coherent detection scheme for THz radiation based on detection of the polarization change of the optical probe beam induced by the THz electric field via the electro-optic Pockels effect in an electro-optic crystal. The field-induced birefringence of the sensor crystal due to the applied electric field (THz wave) modulates the polarization ellipticity of an optical probe beam that passes through the crystal. The ellipticity modulation of the optical beam can then be polarization analyzed to provide information on the amplitude of the applied electric field. A balanced detection system analyzes a polarization change from the electro-optic crystal and correlates it with the amplitude of the THz electric field. A variable time delay between the THz radiation pulse and the optical probe pulse is typically provided by changing the relative length of the beam path between the THz radiation pulse and the optical probe pulse. This technique is sometimes referred to as a xe2x80x9cpump-probexe2x80x9d sampling method. FS-EOS gives a signal directly proportional to the THz electric field. Because the EO effect is almost instantaneous on the THz time scale, the detection bandwidth is much higher than that of a PDA.
In FS-EOS, the choice of sensor crystals is determined by the matching between the phase velocity of the THz wave and the group velocity of the ultrafast probe pulse. A preferred optical source for the generation of THz waves is an ultrafast Ti:sapphire laser that has an average power of about 0.5 W, a pulse duration of about 100 fs, and a center wavelength of about 800 nm. For a THz-TDS system using a common Ti:sapphire ultrafast laser, zinc telluride (ZnTe) is a preferred sensor crystal for EO sampling, because the velocity-matching condition is well satisfied in ZnTe at an optical wavelength of 822 nm, which also makes ZnTe a preferred electro-optic crystal for THz optical rectification generation. A preferred orientation to generate and detect THz waves in a ZnTe crystal is a  less than 110 greater than  cut. If optical sources with different wavelengths are used, the phase matching condition may be different, meaning that other electro-optical crystals may be more favorable. For example, GaAs is more favorable for an 1.5 xcexcm optical beam and InP is more favorable for an 1.3 xcexcm optical beam.
According to Abbe""s law, the spatial resolution that can be achieved when imaging with electromagnetic waves is limited by the wavelength of the employed radiation. The diffraction limit to spatial resolution is not fundamental, however, but rather arises from the assumption that the light source is typically many wavelengths away from the sample of interest. With the lateral scanning of a light source in close proximity to a sample, one can generate an image at a resolution that is functionally dependent on only the source size and the source-to-sample separation, each of which can, in principle, be made much smaller than the wavelength of the employed radiation.
Conventionally, in near-field microscopy, the light incident upon one side of an optically opaque screen is transmitted through a subwavelength-diameter aperture to realize a tiny source. Near-field microwave and optical microscopy is already well known. The concept of near-field microscopy has also been adopted to improve upon the diffraction-limited spatial resolution of scanning THz wave imaging systems, in which the peak frequency of THz radiation is generally 0.5 THz. One near-field method is to use a THz source comprising a tapered metal tube with a nearly circular aperture of less than 100 xcexcm diameter. Another near-field method is to place the sample that is to be imaged close to the THz emitter. One disadvantage of the tapered metal tube is, however, that the high-pass filtering of the THz signal due to the waveguide effect of the tapered metal tube not only decreases the THz signal, but also seriously limits the transmitted THz bandwidth. Another disadvantage is that the spatial resolution is determined by the spot size of the optical pump beam and the finite thickness of the electro-optic crystals needed for relatively strong THz generation. If the spot size of the optical beam is too small, two-photon absorption may limit the generation efficiency through the saturation effect.
Consequently, there is still a need in the art for methods of and systems for improving the spatial resolution of THz imaging that avoid some of the disadvantages of currently used systems.
One aspect of the invention comprises a system for using terahertz (THz) radiation to produce an image of an object. The system comprises a mechanism for providing an optical pump pulse, an optical probe pulse, and an optical gating pulse with a variable delay time between the optical pump pulse and the optical probe pulse. The system also comprises a THz emitter for emitting a beam of THz radiation when activated by the optical pump pulse, and a mechanism for chopping the optical gating pulse on and off. The system also comprises a layer of semiconductive material that is either part of the object itself or a discrete layer between the object and the THz beam. The system further includes structure for focusing the optical gating pulse and the THz beam on the layer of semiconductive material so that the gating pulse illuminates a gating pulse focal spot on the layer of semiconductive material. The gating pulse focal spot has a diameter effective to cause measurable modulation in transmission of the THz beam through the layer of semiconductive material when the gating pulse is on as compared to when the gating pulse is off, creating alternating modulated THz beams that illuminate the object.
A THz receiver, positioned to receive the alternating modulated THz beams after reflection from or transmission through the object, modulates the optical probe pulse with the alternating modulated THz beams to create corresponding modulation in the polarization ellipticity of the optical probe pulse. An element converts the modulation in the polarization ellipticity of the optical probe pulses to intensity modulation. Another element converts the intensity modulation to electronic information. Still another element receives the electronic information and produces an image of the object from the electronic information.
The THz emitter and the THz receiver may each comprise an electro-optic crystal or a photoconductive antenna. The mechanism for providing the optical pump pulse, optical probe pulse, and optical gating pulse may comprise a laser source. One delay stage may be used to provide a variable delay time between the optical pump pulse and the optical probe pulse, and another delay stage may be used to provide a variable delay time between the optical gating pulse and the optical pump pulse. A chopper may be used to turn the optical gating pulse on and off. A lens may be used to focus the optical gating pulse on the object.
Another aspect of the invention includes a method for using THz radiation to generate an image of an object. The method comprises providing an optical pump pulse, an optical probe pulse, and an optical gating pulse with a variable delay time between the optical pump pulse and the optical probe pulse. The method further comprises activating a THz emitter with the optical pump pulse to emit a beam of THz radiation, and chopping the optical gating pulse on and off. The optical gating pulse and the THz beam are focused on a layer comprising semiconductive material that is part of the object itself or a discrete layer placed between the object and the THz beam, so that the gating pulse illuminates a gating pulse focal spot on the layer comprising semiconductive material.
The gating pulse focal spot has a diameter effective to cause measurable modulation in transmission of the THz beam through the layer comprising semiconductive material when the gating pulse is on as compared to when the gating pulse is off, creating alternating modulated THz beams which illuminate the object. The optical probe pulse is modulated with the alternating modulated THz beams in a THz receiver, positioned to receive the alternating modulated THz beams reflected from or transmitted through the object, to create corresponding modulation in the polarization ellipticity of the optical probe pulse. The modulation in polarization ellipticity of the optical probe pulse is converted to an intensity modulation, which is detected and converted to electronic information. The electronic information is then received and processed to produce the image of the object.
The measurable modulation in transmission of the THz beam through the semiconductor may be caused by generation of photocarriers within the semiconductor, particularly where the semiconductor comprises gallium arsenide. The measurable modulation in transmission of the THz beam through the semiconductor may also be caused by a temperature effect within the semiconductor, particularly where the semiconductor comprises silicon.
Another aspect of the invention includes a method of improving spatial resolution of a pump-probe THz imaging system to produce an image of an object comprising a semiconductive material. The improvement comprises the step of providing a chopped optical gating beam focused on the object in a gating pulse focal spot, the gating pulse focal spot having a diameter effective to cause measurable modulation in transmission of a THz beam through the object when the gating pulse is on as compared to when the gating pulse is off, creating alternating modulated THz beams for detection and processing.
Yet another aspect of the invention includes a method of improving spatial resolution of a pump-probe THz imaging system to produce an image of an object using a THz beam. The improvement comprises the steps of (a) placing a layer of semiconductive material between the object and the THz beam, and (b) providing a chopped optical gating beam focused on the layer of semiconductive material in a gating pulse focal spot. The gating pulse focal spot has a diameter effective to cause measurable modulation in transmission of the THz beam through the layer of semiconductive material when the gating pulse is on as compared to when the gating pulse is off, creating alternating modulated THz beams for detection and processing.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.